1. Field of the Invention
This invention relates to a data reproducing device, and particularly to a data reproducing device for detecting a reproducing signal from a magnetic recording medium like a magnetic disk by a magnetic head to reproduce data.
2. Description of the Related Art
In a digital audio tape recorder (DAT) or a magnetic disk device for recording data on a magnetic recording medium like a magnetic tape or a magnetic disk and reproducing the data therefrom, a magnetic head (so-called ring head) is used as a recording head or a reproducing head. In such a magnetic disk device, for example, a reproducing signal is detected by the magnetic head from the magnetic disk on which data is recorded, and the reproducing signal has its waveform equalized by an equalizer so that the data is reproduced.
This equalizer is normally constituted by an integrating circuit having an integration characteristic with respect to low frequency bands, a differentiating circuit having a differentiation characteristic with respect to high frequency bands, a phase equalizer for changing the phase without changing the amplitude, and a low pass filter (LPF) for passing signals of required frequency bands. The integrating circuit compensates the differentiation characteristic of the magnetic head, and the differentiating circuit compensates the loss due to a gap in the magnetic head. The phase equalizer corrects a phase shift due to the LPF and the phase of the bands.
In the DAT and the magnetic disk device, the waveform of the reproducing signal is equalized by the equalizer of the foregoing circuit structure to reproduce the data. However, the following problem is raised.
When a satisfactory integration characteristic in a low frequency range is obtained with the equalizer constituted by an analog circuit, the resistor or the capacitor constituting the integrating circuit exhibits an impractical resistance or capacitance value. That is, with the equalizer of the analog circuit, a satisfactory integration characteristic cannot be obtained. On the other hand, when the integration circuit is constituted by a digital filter, the order increases excessively, causing a large phase shift. Therefore, the phase equalizer in a subsequent stage becomes complex, and the differentiating circuit cannot perfectly correct the loss due to the gap of the reproducing head.
The circuits constituting the reproducing system, such as the reproducing amplifier and the equalizer, are connected to each other via so-called coupling capacitors for cutting the direct current (DC). These coupling capacitors function as high pass filters (HPFs) for removing low frequency components of the reproducing signal. The phase shift caused by these HPFs adversely affects waveform equalization.
With a device like the DAT employing a rotary head formed of a magnetic head and a rotary transducer, the rotary transducer for receiving the reproducing signal from the magnetic head causes a phase shift of the low frequency bands of the reproducing signal, thus adversely affecting waveform equalization.
That is, a primary LPF having a transfer function H as expressed by the following equation (1) exhibits a phase characteristic .theta. as expressed by the following equation (2). EQU H=.omega..sub.0 /(s+.omega..sub.0) (1) EQU .theta.=-tan.sup.-1 (.omega./.omega..sub.0) (2)
As shown in FIG. 1, if .omega.=0, .theta.=0. If .omega.=.omega..sub.0 (=2.pi.f.sub.0, with f.sub.0 expressing the cut-off frequency), .theta.=-.pi./4. If .omega.=.infin., .theta.=-.pi./2.
On the other hand, a primary HPF having a transfer function H as expressed by the following equation (3) exhibits a phase characteristic .theta. as expressed by the following equation (4). EQU H=s/(s+.omega..sub.0) (3) EQU .theta.=tan.sup.-1 (.omega..sub.0 /.omega.) (4)
As shown in FIG. 1, if .omega.=0, .theta.=.pi./2. If .omega.=.omega..sub.0, .theta.=.pi./4. If .omega.=.infin., .theta.=0. Either with LPF or with HPF, the phase shifts ahead of the cut-off frequency in the low frequency range, with f.sub.0 as the reference.
When a linear circuit is employed, the phase characteristic is such that the phase shifts ahead in the low frequency range do not change with any order, and failure in integration equalization is generated. Also, when the phase of the low frequency range of the reproducing signal is shifted ahead by the coupling HPF or the rotary transducer, the analog circuit can only shift the phase of the low frequency range further ahead by using a high-order phase shifter or a combination of plural primary phase shifters having a transfer function H and a phase characteristic .theta. as expressed by the following equations (5) and (6), wherein if .omega.=0, .theta.=0; if .omega.=.omega..sub.0, .theta.=-.pi./2: and if .omega.=.infin., .theta.=-.pi.. EQU H=(s-.omega..sub.0)/(s+.omega..sub.0) (5) EQU .theta.=tan.sup.-1 ((2.omega.-.omega..sub.0)/(.omega..sup.2 -.omega..sub.0.sup.2)) (6)
Thus, phase compensation is difficult.
Specifically, with a DAT, when the bit rate (frequency) of data recorded on magnetic tape is set to f.sub.r Hz, the cut-off frequency f.sub.0 of the conventional integrating circuit is f.sub.r /128 to f.sub.r /64. About four of the coupling HPFs are used in the reproducing amplifier and the equalizer, and the cut-off frequency f.sub.0 thereof is f.sub.r /512. The cut-off frequency f.sub.0 of the rotary transducer is f.sub.r /1024 to f.sub.r /512.
Accordingly, with a conventional device like the DAT, the phase of the low frequency range of the reproducing signal largely shifts ahead, as shown FIG. 2. The conventional equalizer compensates the phase shift only with respect to frequencies not lower than a predetermined frequency, and does not perform phase compensation with respect to frequencies lower than the predetermined frequency. Stated differently, with the conventional DAT or magnetic disk device, phase compensation of the low frequency range is not performed satisfactorily. Therefore, an error correction code with high error correction ability, that is, high redundancy is required, even though an increase in the density of a magnetic recording medium like the magnetic disk is difficult.